TM 9-25410-10.Lens Bench - Continued(9)Eccentricity. A lens is said to be concentric when the geometrical or mechanical axis of the lenscoincides with optical axis. When a lens is concentric, there will be no perceptible movement of theimage when the lens is rotated about its geometrical axis. Conversely, when these two axes do notcoincide, the lens is eccentric, and the amount of eccentricity is proportional to the movement of theimage when it is rotated about its geometrical axis. When the optical bench is used to measure theeccentricity of a lens, the regular chuck in the nodal slide is replaced by a special chuck, which willpermit the lens to be rotated freely on an axis parallel to the ways of the bench and coincidental with theaxes of the collimator and microscope. When a lens is mounted in the chuck and is rotated through 360degrees, the image of the intersection of the cross lines will trace a circle against the target, as seen inthe observing microscope. The radius of this circle is directly proportional to the eccentricity of theelement under observation. A convenient target for measuring the actual amount of eccentricityconsists of a group of twelve concentric circles whose centers are at the center of the target and whoseradii will indicate a series of deviations from 1 to 12 minutes for the particular focal length of thecollimator being used.10-13/(10-14 blank)